I know that Magikarp on its own has a 65% encounter rate, but I am interested in knowing what are the odds of Magikarp appearing specifically at level 15 when you reel in one with an Old Rod.
The encounter rate of finding a level 15 Magikarp in Dewford Town when fishing with the Old Rod should be 32.5%, on the assumption that the chance of getting a level 10 or level 15 Magikarp is equal. You've got a 65% chance to encounter a Magikarp, and a 50% chance for it to be level 15. They cannot be encountered on any of the levels in between, according to Bulbapedia.
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